A study on calculation method of duct streaming from medical linac rooms

The leakage radiation dose from medical linac systems must be suppressed to prevent public exposure to radiation. Therefore, leakage dose through ducts or sleeves installed in shielding walls need to be evaluated because radiation can pass through these without undergoing scattering or attenuation. However, conventional methods for calculating radiation streaming have not been sufficiently evaluated for the leakage dose from the ducts in mazes. In this study, the method provided by the Japan Radiological Society for neutrons and X-rays, Nakamura and Uwamino’s formula for neutrons, the method described by the IAEA Safety Reports Series No.47 and the method given by McGinley for X-rays were evaluated by comparing the results with a Monte Carlo calculation using MCNP5 in calculation models. The rectangular ducts in these models were located in the maze near the entrance of linac room. Compared with MCNP5, most results for duct streaming obtained using conventional methods for neutrons were within a factor of 2, whereas the results of methods using X-rays were more than a factor of 2. The results for maze streaming using Nakamura and Uwamino’s formula were in good agreement and differed by not more than 16%. Furthermore, the ratios of the X-ray dose at the duct entrance to that at the exit were calculated to investigate X-ray duct streaming with the results of MCNP5 and showed the relationships against the ratio of the opening area of the duct to square length.